Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 57 (p. 1172)

1. Macrophages kill cells by engulfing the ones presenting the appropriate antigen.

2. The complement system is composed of a series of about 20 freely-circulating proteins in the blood. When they encounter an infected cell, they assemble an "attack complex" that creates a pore in the membrane of the infected cell. Water can then enter the cell, ultimately causing it to burst.

3. MHC I proteins are found on all nucleated cells in the body, identifying it to the body as "self," so that when an immunological attack is in progress, the body does not destroy its own cells along with the pathological invaders. MHC II proteins are found only on macrophages, B-cells, and CD4⁺ T cells; they function in identifying each other.

4. Macrophages activate helper T cells by engulfing the invading pathogen and presenting the pathogen's antigens on its surface for the helper T cells to recognize. Macrophages also secrete interleukin-1 that stimulates the helper T cells. Once activated, helper T cells secrete lymphokines that mobilize cytotoxic T cells to fight off the pathogen.

5. IgM functions to attract and persuade the proteins of the complement system in for attack. IgG marks the invading pathogen for phagocytosis.

6. When a B lymphocyte responds to an antigen, it is stimulated to replicate, producing a clone of cells that provides an initial, weak immune response. Some of the cells become memory B cells. The next time the body is invaded by the same pathogen, it is ready to provide a rapid, strong secondary immune response.

7. Lymphocytes create immune receptors out of "gene kits." By combining a smaller number of components several different ways, millions of receptors can be reproduced from only a few hundred copies of different genes.

8. Invertebrates, such as insects, use a negative test to recognize foreign cells. Their immune systems look for their own surface markers. If they fail to find them on the inspected cell, it will attack and engulf the foreign cell. Foreign cells with markers that resemble those of the host will be allowed to survive. Mammalian cells also have "self" surface markers for which their immune systems test. Cells not recognized will be attacked in a complex manner with a considerable variety of cells and actions. Some defense cells are phagocytic and some produce cytotoxins. Some cells secrete chemicals that stimulate other defense cells to attack or multiply. Memory cells are also generated that will make recognition of the invader faster in the future.

9. Autoimmune diseases result when the immune system fails. The self-antigen may be hidden from the immune system, as in Hashimoto’s thyroiditis, or antibodies may be made to nucleoproteins, as in systemic lupus erythematosus.

10. AIDS interferes with the immune response by three means: (1) AIDS-infected cells release progeny viruses that infect other CD4⁺ cells, killing them and releasing more viruses; (2) AIDS viruses cause CD4⁺ cells to secrete a factor that blocks other T cell responses to the viral antigen; and (3) AIDS viruses block transcription of the MHC genes, hindering the recognition and destruction of infected cells.